Abstract The goal of this project is to create a heart valve that can grow with children and never require replacement. Using a biologically-engineered collagenous matrix in the form of a tube that has been shown to grow as a pulmonary artery replacement in lambs, and a novel design for a heart valve made from these tubes and degradable suture that confers durable commissures as well as valve growth potential, we will design, optimize, test, and implant tri-tube valves in a juvenile sheep model to demonstrate valve growth. Aim 1 will use computer-aided design (a fluid-structure interaction model for valve opening and finite element model for the closed valve state) to screen combinations of design parameters (defining coaptation area and geometry) for efficient valve function, Aim 2 will test the optimal designs in a pulse duplicator and validate/refine the CAD models, and Aim 3 will involve studies of the valve in growing juvenile sheep. Long-term studies of pulmonary and aortic valve replacement are designed with measurements to rigorously address valve growth.